The present invention relates to a wavelength tunable optical transmitter and an optical transceiver, and particularly relates to a wavelength tunable optical transmitter and an optical transceiver which operate stably over wide wavelength channels.
Currently, dense wavelength division multiplexing (DWDM) optical transmission for transmitting optical signals in multiple wavelengths by a single optical fiber, which is often used in a backbone system, is an important transmission system that can realize long distance and large capacity optical transmission. For the DWDM optical transmission, the ITU-T standard defines grid wavelengths (wavelength channels) in a 1.55 μm wavelength band for realizing the long-distance optical transmission, of which the wavelength interval (frequency interval) is 0.4 nm (50 GHz) or 0.8 nm (100 GHz).
A wavelength tunable optical transmitter has been developed, which can cover wavelengths of multiple channels by a single optical transmitter. In the wavelength tunable optical transmitter, it is necessary that only an oscillation wavelength is changed in a wavelength interval for DWDM, and other characteristics (optical output power, a modulation characteristic, a transmission characteristic and the like) are kept constant irrespective of a wavelength.
Currently, there are roughly two types of laser modules used for the wavelength tunable optical transmitter for DWDM. One is in a method of using multiple laser arrays, and the other is in a method of changing the temperature of a laser.
The former is an LD array in which multiple distributed feedback laser diodes (DFB-LD) or distributed Bragg reflector laser diodes (DBR-LD), of which the pitches of diffraction gratings are varied such that they oscillate at different oscillation wavelengths from each together in a wavelength interval for DWDM, are integrated in an array form on a semiconductor photonic device. In this configuration, generally, a multi-mode interference (MMI) multiplexer and a semiconductor optical amplifier (SOA) are also integrated in the same device. The MMI multiplexer is to multiplex oscillation light from each of the laser diodes of the LD array, and SOA is to compensate loss of light power in the MMI. When the LD array is used, since the oscillation wavelength can be varied to the number of LDs in the LD array even if the temperature of the semiconductor photonic device is kept constant, many channels can be covered.
An external modulator such as an EA modulator can be integrated in the front of SOA in order to modulate oscillation light having multiple wavelengths from the LD array. However, an integrated external modulator can be operated only within a wavelength range in which the modulation characteristic or the transmission characteristic can be kept constant. Therefore, typically, a lithium niobate (LN: LiNbO3) modulator module, which has small wavelength dependence with respect to the modulation and transmission characteristics, is often provided outside a wavelength tunable laser module.
JP-A No. 2001-144367 describes that a semiconductor laser system in a configuration of a combination of the LD array, a coupler (multiplexer), a single semiconductor optical amplifier, and a single EA modulator is driven by changing offset bias of the EA modulator. As a U.S. counterpart to JP-A No. 2001-144367, U.S. Pat. No. 6,516,017 is given.
The latter is a single DFB-LD integrated with a single EA modulator (EA/DFB: Electroabsorption Modulator Integrated DFB Laser). In this case, the temperature of the EA/DFB device is changed so that the oscillation wavelength is changed to be adapted to the wavelength channel for DWDM.
JP-A No. 2005-045548 describes an optical transmitter in which a semiconductor laser and an electroabsorption modulator are separately subjected to temperature control. In the optical module, the temperature of the semiconductor laser is changed to vary a wavelength, and information on the wavelength is fed back to temperature control of the modulator, thereby a constant characteristic can be obtained without changing driving bias of the modulator.
S. Makino et al., “Wide Temperature Range (0 to 85° C.), 40-km SMF Transmission of a 1.55-μm, 10 Gbits/s InGaAs Electroabsorption Modulator Integrated DFB laser”, OFC2005, PDP14 describes an indium-gallium-aluminum-arsenic (InGaAlAs) base EA/DFB laser which can be used without cooling in a range of 0° C. to 85° C. In addition, the above literature describes that the oscillation wavelength is 1550 nm at 0° C. but about 1560 nm at 85° C.
The semiconductor laser system of JP-A No. 2001-144367 is difficult to be reduced in overall module size. Moreover, since there are many types of functional elements integrated on one device, significant yield reduction may anxiously occur in fabricating the device.
In the optical transmitter of JP-A No. 2005-045548, the semiconductor laser and the electroabsorption modulator are provided as separate elements, and coupling loss necessarily occurs between them. Moreover, since the semiconductor laser and the electroabsorption modulator are separately subjected to temperature control, reductions in size and power consumption are difficult.
Embodiments of the invention use a single EA modulator/laser. In this case, the EA modulator/laser may be used in a wide temperature range, consequently it is necessary to adjust drive conditions of a laser device and an EA modulator device.